The overall objective of this research is to purify tumor antigens of defined specificities from spent culture medium of melanoma cell lines and develop sensitive immunological tests, e.g., radioimmunoassay (RIA) and enzyme immunoassay (EIA), for detection and quantification of such antigens in the serum and other body fluids of cancer patients. Two antigens have been detected in the spent culture medium of a melanoma cell line that are immunogenic in the host. One antigen is of fetal origin and has been termed as fetal antigen (FA). FA is a glycoprotein of 60 to 70 kilodaltons. It is expressed by a majority of tumors of various histological types and fetal tissues as detected by the competitive EIA. The incidence of natural antibodies to FA in serum samples from cancer patients is almost equal to those from normal volunteers. The other antigen has been observed to be preferentially expressed by melanoma cells and rarely by other tumor cells. Thus, this antigen has been designated as MAA. Normal cells either do not express MAA-like antigen or, if they do, the amount is below detectable level of the competitive RIA. Thus far, 69 various normal tissues have been negative for MAA. The MAA has been tentatively characterized as a lipoprotein with a Mr of approximately 180 to 190 kilodaltons. This antigen, as opposed to FA, is heat sensitive. The MAA activity is destroyed by treatment with proteolytic enzymes and not by glycosidic enzymes. Presence of carbohydrates could not be detected by a number of colorometric methods, and this antigen does not bind to a number of lectins. MAA-like activity has been detected in circulating immune complexes isolated from plasma of melanoma patients. (2)